Unwrap torso part creates a messed up UV map
Fiddeling around and examing a large number of related topics I found the solution by myself.

It seems the mesh is pinned (to whatever) and has to be freed before.


Select the mesh in edit mode
Go to UV mode/window and select all (A)
Unpin the mesh in UV mode (Alt P)
Redo the unwrap in UV mode (E)


Voila!